A 2-DIMENSIONAL TOMOGRAPHIC STUDY OF THE CLIPPERTON TRANSFORM-FAULT

From the marine refraction data recorded on five instruments during th e Clipperton Area Seismic Survey to investigate Compensation (CLASSIC) experiment in 1994 we construct a compressional velocity model for a 108 km long profile across the Clipperton transform. We apply a new se ismic tomography code that alternates between ray tracing and lineariz ed inversions to find a smooth seismic velocity model that fits the ob served refraction travel times. The solution to the forward ray-tracin g problem is a hybrid of the graph (or shortest path) method and a ray -bending method. The inversion is performed with least squares penalti es on the data misfit and first derivatives of the seismic structure. Starting with a one-dimensional compressional velocity model for ocean ic crust, the misfit in the normalized travel time residuals is reduce d by 96%, decreasing the median travel time residual from 110 to 25 ms . The compressional velocity structure of the Clipperton transform is characterized by anomalously low velocities, about 1.0 km/s lower than average, beneath the median ridge and parallel troughs of the transfo rm domain. The low compressional velocities can be explained by an inc reased porosity due to fracturing of the oceanic crust. We found crust al thicknesses of 5.6-5.9 km under the transform fault to produce the best fit of the PmP phase arrivals and Pg/Pn crossovers. Since the cru st is not thin beneath the transform parallel troughs and the velocity anomaly is not confined to the median ridge, we find uplift by serpen tinite diapirs unlikely as an explanation for the relief of the median ridge. A median ridge that is the result of brittle deformation due t o compression across the transform domain is, however, compatible with our results. The upper crust is thicker to the north of the transform than to the south, which is likely a consequence of the contrast in t emperature structure of these two spreading segments.